1. Field of the Invention
The present invention relates to a dishwasher comprising a washing tub having a sump, at least one spraying nozzle located within the washing tub, means for feeding fresh water into the sump, a circulation pump for circulating process water from the sump to the spraying nozzle, and a conductivity sensor for measuring the conductivity of the fresh and/or the process water.
2. Description of Related Art
Such a dishwasher is known from U.S. Pat. No. 4,211,517, which discloses a commercial dishwasher that it provided with a conductivity sensor that is located in the sump and which is used to measure the pH-level in the sump so as to control the supply of detergent during a washing cycle. The dishwasher disclosed in U.S. Pat. No. 4,211,517 is disadvantageous, because it only allows to measure the conductivity of the process water. Measurements of the conductivity of the fresh water, which could be used to determine the water hardness or to provide for a calibration of the set-point level of the conductivity to be achieved, are not possible.
In order to overcome such problems, it was suggested in EP 0 686 721 B1 for a washing machine to provide, in addition to a first conductivity sensor that is arranged in the sump of the washing tub and which is used to measure the conductivity of the process water, a second conductivity sensor that is provided in the water inlet line. While in this manner it is possible to measure both the conductivity of the fresh water and of the process water, the solution provided for in EP 0 686 721 B1 has the disadvantage that it requires the provision of two conductivity sensors, which thus adds to the complexity and costs of the system.
Furthermore, from EP 1 688 529 A1 there is known a washing machine having a water intake, which is connected to a detergent drawer that is located at the top of the washing machine so as to be above the water level within the washing tub. When the water intake is activated, washing powder that has been provided within the detergent drawer is flushed out and is passed via a conduit into a washing drum, which can be rotated within the tub. In order to be able to evaluate whether the washing powder has been completely flushed out of the detergent drawer into the washing tub, the conduit leading from the detergent drawer to the washing tub is equipped with a conductivity sensor and with a turbidity sensor. During rinsing the washing powder out from the detergent drawer both the conductivity sensor and the turbidity sensor continuously provide a measuring signal, which signals during flushing the detergent drawer change until the washing power has been completely flushed out of the detergent drawer. While thus EP 1 688 529 A 1 employs the conductivity sensor solely to detect whether the conductivity changes, in this arrangement it is not possible to qualitatively measure the conductivity of the freshwater or the process water, since the water flowing through the conductivity sensor in any event has to pass the detergent drawer, which at any time of the measurement may contain an unknown amount of washing powder.